1. Field of the Invention
The present invention relates generally to an apparatus and method for managing a virtual cell in an OFDM mobile communication system, and in particular, to an apparatus and method for managing resources of a virtual cell.
2. Description of the Related Art
In general, orthogonal frequency division multiplexing (hereinafter referred to as “OFDM”) can be defined as a two-dimensional access technology, a combined technology of time division access (TDA) and frequency division access (FDA). Therefore, in OFDM data transmission, OFDM symbols are separately carried on corresponding subcarriers (or subchannels).
OFDM has high spectrum efficiency because spectra of subchannels overlap with one another while maintaining mutual orthogonality. A modulation/demodulation scheme can be realized with an efficient digital device since OFDM modulation/demodulation is realized by inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). In addition, the OFDM technology, because it is robust against frequency selective fading or narrowband interference, is effective for the current European digital broadcasting transmission and high-speed data transmission, and has been adopted as standard specification for a high-capacity wireless communication system, such as IEEE 802.11a, IEEE 802.16a, and IEEE 802.16b.
OFDM is a type of a multicarrier modulation (MCM) technology that converts a serial input symbol stream into parallel symbols and then modulates the parallel symbols with a plurality of orthogonal subcarriers before transmission.
A system supporting MCM was first applied to high frequency wireless communication for military use late in the 1950's, and a system supporting OFDM (hereinafter referred to as “OFDM system”), in which a plurality of orthogonal subcarriers overlap with one another was developed in the 1970's. There was a limitation on application of OFDM to an actual system, because orthogonal modulation among multiple carriers must be realized. However, as discussed by Weinstein et al. in an article entitled “Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform” in IEEE Transactions on Communications, Volume: 19 issue 5, October 1971 (pages: 628–634) OFDM modulation/demodulation can be efficiently performed using discrete Fourier transform (DFT), the OFDM technology has developed rapidly. In addition, as a method of using a guard interval and inserting a cyclic prefix guard interval is known, it is possible to reduce the influence of multipath fading and delay spread on the system. As a result, OFDM is widely applied to digital transmission technologies such as digital audio broadcasting (DAB), digital television (TV), wireless local area network (W-LAN), and wireless asynchronous transfer mode (W-ATM). That is, OFDM has not been widely used due to its high hardware complexity, but the recent development of various digital signal processing technologies including FFT and IFFT make it possible to realize OFDM. The OFDM technology, being similar to the conventional frequency division multiplexing (FDM) technology, is characterized by transmitting high-speed data while maintaining orthogonality among a plurality of subcarriers, thereby securing optimum transmission efficiency. In addition, OFDM has high frequency efficiency and is robust against multipath fading, thus securing optimum transmission efficiency during high-speed data transmission. Particularly, since frequency spectra overlap with one another, OFDM has high frequency efficiency and is robust against frequency selective fading and multipath fading. Further, according to OFDM, inter-symbol interference (ISI) can be reduced using a guard interval and an equalizer can be simply designed by hardware. In addition, OFDM is positively applied to a communication system because it is robust against impulsive noises.
An existing OFDM mobile communication system having the above-stated characteristics fixes time and frequency channels allocated to users, like an OFDM-TDMA (Time Division Multiple Access) or OFDM-FDMA (Frequency Division Multiple Access) cellular mobile communication system. That is, in the existing OFDM mobile communication system, a user supporting the OFDM technology and the TDMA or FDMA technology transmits OFDM data by TDMA or FDMA. In this case, the same frequency bands are reused by a plurality of cells in order to increase frequency efficiency. The extent of the frequency reuse is determined by a frequency reuse factor. Commonly, the frequency reuse factor becomes 3, 4, or 7. Therefore, frequency reuse efficiency is not that high (a frequency reuse factor is larger than 1) due to a fixed channel allocation technology, and a fixed subchannel allocation technology shows a poor bit error rate (BER) due to frequency selective fading.
In addition to a system employing the fixed channel allocation, wideband wireless access technologies based on a method of equalizing the influence of interference, such as a band division multiple access (BDMA) technology and a multicarrier code division multiple access technology have been proposed. The interference equalization is achieved by diversity effect of interference occurring due to inter-cell random frequency hopping and spread spectrum technologies. The interference equalization technology shows higher capability than the fixed channel allocation technologies such as OFDM-TDMA and fixed OFDM-FDMA. However, the interference equalization technology cannot completely realize advantages of multicarrier modulation such as multiuser diversity and adaptive resource allocation with channel information in base stations (BSs). An interference avoidance technology such as dynamic channel allocation can show 2 or 3 times higher capability than the interference equalization technology in terms of frequency efficiency. Therefore, a combination of the OFDM technology and a dynamic subchannel allocation technology based on a multi-antenna technology, adaptive modulation, and an interference avoidance technology with low complexity, remarkably reduces the influence of deep fading and co-channel interference (CCI) while increasing frequency efficiency and system capacity.
FIG. 1 illustrates a method of reusing frequencies of each cell in a general mobile communication system supporting fixed OFDM-FDMA (hereinafter referred to as “fixed OFDM-FDMA mobile communication system”). Specifically, FIG. 1 illustrates an example of carrier frequencies used in respective cells, wherein a carrier frequency used in each cell is reused by other cells except in directly adjacent cells. Therefore, in FIG. 1, a frequency reuse factor becomes 3. Herein, an allocated bandwidth is divided into three bandwidths, and a deterministic scheme, i.e., a fixed channel allocation technology, is used in each cell.
FIG. 2 illustrates a transmission/reception scheme of each cell in a fixed OFDM-FDMA mobile communication system. Referring to FIG. 2, user data User#1, User#2, . . . , User#K to be transmitted to a plurality of users is applied to a fixed subcarrier allocator 210. The fixed subcarrier allocator 210 allocates at least one fixed subcarrier to the user data to be transmitted to each user (Fixed Subcarrier Allocation). The user data allocated at least one fixed subcarrier is provided to a modulation and IFFT conversion block 212, and the modulation and IFFT conversion block 212 modulates the user data by a predetermined modulation scheme and then performs IFFT conversion on the modulated user data. The IFFT-converted user data is provided to a cyclic prefix addition and parallel/serial (P/S) conversion block 214, and the cyclic prefix addition and P/S conversion block 214 inserts a cyclic prefix guard interval into the user data, converts the parallel user data into one serial user data stream, and then transmits the user data stream through a transmission antenna.
User data transmitted from each user after being processed in the above-stated process is received through a reception antenna. The received user data is provided to a cyclic prefix cancellation and serial/parallel (S/P) conversion block 220, and the cyclic prefix cancellation and S/P conversion block 220 converts the received user data into parallel user data, and cancels cyclic prefix guard intervals inserted in the user data. The user data output from the cyclic prefix cancellation and S/P conversion block 220 is provided to an FFT block 222, and the FFT block 222 restores the user data and provides the restored user data to a demodulator 224. The demodulator 224 demodulates the restored user data and generates kth user data.
In the conventional transmission/reception scheme for the fixed OFDM-FDMA mobile communication system described in conjunction with FIG. 2, although the required number of calculations is reduced, frequency and power efficiencies are not high. Therefore, the fixed OFDM-FDMA mobile communication system has the following disadvantages.
First, if several subchannels of a particular user experience deep fading or interference due to fixed channel allocation being increased, BER performance will be poor.
Second, when a frequency reuse factor is 3, frequency efficiency is low. That is, because traffic for each cell is independent, a cell which is concentratedly accessed by users may reach its limitation on an amount of supportable traffic, unlike the adjacent cells, which are not concentratedly accessed by users. In this case, the cell that has reached its limitation on supportable traffic is unable to borrow the channels unused in the adjacent cells according to circumstances. Therefore, there are demands for a wireless access technology based on an adaptive resource allocation algorithm in which a frequency reuse factor approaches 1.